1. Field of the Invention
The invention relates to an electrical device and method and, more particularly, to capacitance and inductance.
2. Brief Description of Prior Developments
A capacitor can be used for a power supply for an apparatus, such as a Switch Mode Power Supply (SMPS) or a xenon flash for a camera for example.
In a switch mode power supply (SMPS), energy flow is controlled with power semiconductors that are continuously switching ON and OFF with high frequency. The “switchers” offer greater efficiency compared with linear supplies because an ideal switch does not dissipate power. They also have smaller size and lighter weight due to reduced passive components size and lower heat generation.
Switch mode operation contributes to lower power losses—when a switch is “off” the current through it is near zero, when it is “on” the voltage across it is low. However, it introduces additional electrical noise (EMI) whose acceptable limits vary with the frequency. The switcher's frequency is normally selected based on size, efficiency, cost and EMI considerations.
To reduce noise of SMPS an input noise reduction filter is used in addition to a SMPS input capacitor. This is achieved by adding another capacitor and a resistor or inductance between the two capacitors. The additional capacitor can also work as an input capacitor, but the resistor or inductance has no other use other than filtering the noise.
In a capacitor a serial inductance has an harmful impact. That is why the terminals are design so that the inductance is minimized. The serial inductance and serial resistance, ESR, is considered a negative characteristic of a capacitor.
A three-terminal capacitor consists of two active electrodes surrounded by a third, or shield, conductor. The direct capacitance between the two active electrodes is the capacitance of interest, and, when shielded leads are used, it is independent of the location of all other conductors except the shield.
A common construction of capacitors is use of windings as plates. A winding roll tends to have inductance, and in order to reduce the inductance the terminals are connected in the center of the electrodes, so the current flowing into and from the ends of the electrodes compensate their magnetic fields; thus reducing the inductance.